Polymerization of olefinic hydrocarbons



May 24 1949. w. B. SHANLEY POLYMERIZATION OF OLEFINIC HYDROCARBONS 2 Sheets-Sheet 1 Filed April 23, 1947 ll mm -m mbmk N tmmsmmzob INVENTOR. Mlllklrrlflfikazzkg BY 2 Sheets-Sheet 2 W. B. SHANLEY POLYMERIZATION Of OLEFINIC HYDROCARBONS okqzotoqmm Filed April 23, .1947

INVENTOR. b/zY/l'avl afiltazzlg aivzey- Patented May 24,

umrsogsrarss A PArcNr orrlcs I POLYMERIZATION F owl-mic maoosanons William B. Slianley, San Marino, Calli., asslgnor r to Universal Oil Products-Company, Chicago,

111., a corporation of Delaware Application April 23, 1947, Serial No. 743,425

merization processes operated in conjunction with petroleum cracking plants to produce valuable motor fuel from the gases formed incidental to cracking operations.

Polymerization of olefin-containing gases is carried out commercially by both thermal and catalytic means. The former method which is operated at relatively high temperatures and pressures give substantial yields of aromatic hydrocarbons as well as olefins, while the catalytic polymerization is operated at much lower temperatures and generally at lower pressures to pro,- duce an olefinic liquid product. In the catalytic process a type of catalyst which has been used with commercial success is the solid phosphoric acid catalyst such as described in U. S. Patent No. 1,993,513 and others. In this process, olefincontaining hydrocarbon gas mixtures are passed through stationary beds or sections of granular material consisting essentially of prepared particles made by incorporating a phosphoric acid with a relatively inert and generally siliceous material to produce a paste which is either calcined to produce a cake that is ground and sized to recover the catalyst particles or the paste is extruded to produce particles which are later cal- 14 Claims. (01. 260-68315) line of flow through the series of catalyst reactors and this pressure dropis another factor which must be considered in a successful operation of such a plant to produce high yields of desired products. The present process is an improvement in processes for polymerizing normally gaseous olefinhydrocarbons to form liquid polymers therefrom while utilizing solid granular catalyst, particularly the solid phosphoric acid catalyst described briefly hereinbefore, although the improved process will'find ready application when other solid polymerizing catalysts are employed, such as copper pyrophosphate, activated clays, silica-alumina composites, and other comcomprises contactin an olefin-containing hydrocarbon fraction at polymerizing conditions in the presence of a solid polymerizing catalyst contained in a series of catalyst sections of increasing thickness in the direction of flow. cooling between catalyst sections by recycling thereto a I hens, and recovering said polymers.

cined. In either case prepared particles are subjected to controlled drying operations to vary the proper composition of active polymerizing acid in respect to its degree of hydration.

In commercial use, it is customary to operate with a number of polymerizing chambers of substantially the same size filled with substantially the same volume of this type of polymerizing catalyst. It is also customary to operate such catalyst towers in series or in parallel and to partially cool the eilluent from one reactor before charging it to the next reactor in the series in order to prevent the exothermic heat of reactionfrom causing an excessive rise in temperature which may result in dehydration and coking of the catalyst.

Obviously, there is a pressure drop along the Another embodiment of this invention relates to an olefin polymerization process which comprises contacting an olefin-containing hydrocarbon fraction at polymerizing conditions oi temperature and pressure in the presence of a solid phosphoric acid catalyst contained in a plurality of catalyst sections of progressively increasing thickness in the direction of flow, cooling between catalyst sections by recycling thereto a portion of .the total efliuent from said cata-- lyst sections including unconverted olefin-containin fraction and polymers, separating the unrecy'cled eiiiuent into polymersand unconverted hydrocarbons, and recovering said polymers.

. butylene) Temperature control during polymerization of olefin-containing feed stocks containing 50 to 60 per cent of olefins is relatively diflicult in a chamber-type polymerization unit by the methods used heretofore because of the high exothermic heat of the polymerization reaction. In order to maintain a temperature of from about 400 to about 500 F. in a chamber-type polymerization reactor, it is necessary to limit the olefin content of the charged hydrocarbon fraction to between about and about 35 per cent and preferably from between to per cent. When charging stocks are produced which contain more than 35 per cent of polymerizable .olefins, temperature control is generally efiected by recycling a portion of the substantially deolefinized exit gas such as propane-butane fraction and commingling it with the olefin-containing charging stock so that the combined feed to the polymerization reactors contains, for example, from 20 to 25 mole per cent of polymerizable olefins (propylene and Obviously, when processing a feed stock containing 60 per cent of polymerizable olefins, it is necessary to recycle a large amount of deolefinized exit gas and to pass through the polymerization catalyst zone an amount of combined feed which is about six times the quantity of fresh feed. In this type of operation, the fractionator to which the efliuent from the polymerization reactors is directed must handle a much larger volume of material than would be thecase if less de-olefinized gas were recycled. Thus the fractionator which must handle total reactor effiuent would have to be approximately six times the size of the fractionator which would be required if it were possible to process the fresh feed without dilution by recycle stock. Because of the need for extra fractionation equipment, a chamher-type polymerization plant loses its low cost feature when operating on feed stocks containing more than about 35 mole per cent of polymerizable olefins.

This situation is improved and the utility of the chamber-type polymerization unitis extended by employing a combined modification in plant design and operating procedure as set forth herein. According to this invention, improved control of the polymerization reaction is obtained by limiting the depth or thickness of the :in'itial catalyst bed or section, of the polymerizathe first catalyst bed. Excessive temperature rise in the first catalyst bed results in catalyst dehydration, formation of coke, or other heavy hydrocarbonaceous materials, high pressure drop, and eventual plugging of the first catalyst reactor. By thus obtaining control of the rise in temperature that occurs in the first bed or section of catalyst, it is possible thereafter to control the temperatures of subsequent catalyst beds by controlling their depths or thicknesses and also by the injection of liquid quenching stock, preferably total efliuent from the catalyst reactors, said liquid quenching stock being directed between catalyst beds or between catalyst reactors. By means of thismethod of limiting the depths of the different catalyst beds, and of recycling a portion of the total eiiluent, it is possible to polymerize a substantial proportion of the olefins from feed stocks containing up to about per cent of polymerizable olefinsusing reasonable amounts of quenching stock. Other suitable valve 2| to reactor 22.

quenching stocks include portions of the fresh feed stock to the polymerization zones and also fractionator overhead which is essentially a C3-C4 fraction of low olefin content. Thus by employing the process of this invention, the utility of the chamber-type polymerization plant can be extended so that it can be used economically in combination with catalytic cracking operations, which produce gases of high olefin content.

The character of the present process is indicated in more detail by reference to the attached Figures 1 and 2 which show diagrammatically in general side elevations two arrangements of interconnected elements in which operations falling within the scope of this invention may be carried out. The drawings, however, do not represent all of the modifications of this invention that may be employed.

Referring to Figure l, a charging stock, for example, a so-called C3-C4 fraction containing propane, propylene, butanes and butylenes is introduced through line I and valve 2 to pump or compressor 3, which discharges through line 4 and valve 5 to coil 6 which receives heat from heater 1 and then passes through line 8, valves 9 and ID to reactor ll containing a bed or section or particles of a solid polymerizing catalyst maintained at a temperature of from about 400 to about 500 F. and at a pressure of from about 100 to about 2000 pounds per square inch. A portion of the C3-C4 fraction being discharged by pump or compressor 3 through line 4 may be directed therefrom through line I2 containing valve l3 to heat exchanger I4 which receives heat from the reaction products of the process as hereinafter set length that the catalyst bed therein has a thickness of 2 feet while reactors i9, 22 and 25 are of such lengths that the catalyst beds therein have thicknesses or depths of 3, 5 and 8 feet. From reactor II the mixture of polymers and unconverted charging stock resulting from the polymerization reaction therein is directed through line I! and valve l8 to reactor l9 and the effluent from this reactor is passed through line 23 and From reactor 22, the mixture of unconverted hydrocarbons and polymers is passed through line 23 containing valve 24 to reactor 25. Also line 8 is connected through by-pass line 26 containing valve 21 to line H and thence to reactor l9 so that the hydrocarbon mixture charged to the process may be passed directly to reactor IS in case reactor Ii becomes plugged or otherwise rendered inoperable by accidental misoperation as by the admission thereto of liquid water with the steam added to prevent dehydration of the catalyst as hereinafter set forth.

From reactor 25, the eilluent comprising olefin polymers and unconverted C3-C4 fraction is directed therefrom through line '28 and valve 29 to fractionator 30. A portion of the mixture of polymers and unconverted C3-C4 fraction is 38 into line 23 by which the effluent from reactor 22 is conducted to reactor 25. Another portion of the material contained in header 3'! is passed through branch line 39 and valve 49 into line 29. The mixture of polymers and unconverted charging stock is thus recycled'and blended with the polymerization products from each reactor before being charged to the next reactor in the series of polymerization reactors in such amounts as to maintain the polymerization temperature between about 400 and 500 F.

when employing solid phosphoric acid polymerization catalyst, the hydrocarbon containing mixtures that are directed to the several reactors of the series such as reactors il, i9, 22 and 25 as herein set forth are also commingled with controlled amounts of steam or water-vapor in order to maintain the catalyst at a desired state of hydration and thereby to assist in maintaining catalyst activity at a relatively high value for long periodsof time. Steam or water vapor is accordingly directed under pressure through header 4i, valve 42 to line 23 and thence to reactor 25. Header 4i is also connected through branch line 43 and valve 44 to line 8, through branch line 45 and valve 46 to line I! and through branch line 41 and valve 48 to line 29. Steam or water vapor is thus commingled with the mixture of hydrocarbons directed to each of the catalyst reactors ll, [9, 22 and 25. The amount of steam or water vapor (from about 0.2 to about 3.5 mole per cent of the hydrocarbon charged) directed recycled) is directed, is provided with reboiler coil 49 which is suppliedwith heat from a source not illustrated in the drawing and is employed to efiect a separation between the normally gaseous and normally liquid constituents of the material charged thereto. From the top of fractionator 39 a C3-C4 fraction of relatively low olefin content is directed through line 50 and valve 5| to ..condenser 52, from which the resultant mixture of liquid and vapors is directed through rundown line 53 and valve 54 to receiver 55 provided with gas release line 56 containing valve 51 and With liquid drawofE line 58 containing valve 59. A portion of the liquefied material contained in receiver 55 is withdrawn therefrom through line 60 and valve Bl to pump 62 which discharges through line 63 and valve 64 to near the top of fractionator 39, to assist in controlling the temperatures therein. From the bottom of fractionator 39, the

normally liquid polymers substantially free from unconverted C3-C4 hydrocarbons, are discharged through line 65 and valve 66 to storage or to use not illustrated in the diagrammatic drawing.

Alternative polymerizing equipment having a catalyst reactor provided with a plurality of catalyst beds of difi'erent and increasing thickness or depth in the direction of'fiow through said reactor is illustrated diagrammatically in the attached Figure 2. According to this modification of the invention an olefin-containing hydrocarbon fraction, for example, a C3-C4 fraction of parafllns and olefins is directed through line 61 and valve 83 to pump or compressor 89 which discharges through line 10 and valve H to heating coil 12 which receives heat from heater l3 and from which the heated hydrocarbon mixture'is passed through line 74 and valve 15 to reactor 18, containing a plurality of catalyst beds or sections indicated diagrammatically as four, each 01' these catalyst beds or sections being supported by a screen or other suitable means indicated in the drawing by screens 71, I8, I9, and 80. These screens or other supporting means are spaced at such intervals in reactor I6 that the catalyst beds or sections supported thereon are relatively thin near the inlet of the reactor and are progressively thicker near the exit of said reactor. For example, catalyst beds supported by screens TI, 18, I9 and may be arranged to have the respective thicknesses of 2 3, 5 and 8 feet.

Whenusing solid phosphoric acid catalyst, it is generally advisable to commingle the charging stock with regulated amounts of steam or water vapor in order to maintain the hydration state of the catalyst necessary for high polymerizing activity and long catalyst life. Thus, in order to provide for hydration of the catalyst, steam under pressure is introduced through header 8|, containing valve 82, to the last catalyst bed or section of reactor 16. Hydration of the catalyst in the other reactor sections of reactor 16 is maintained by directing a position of the steam or water vapor from header 8| through branch line 83 containing valve 94, branch line 85 containing valve 86, and branch line 81 containing valve 88.

From the exit end of reactor 16, a portion of the resultant mixture of polymers and unconverted C3-C4 fraction is discharged through line 89 and valve 90 to fractionator 9i and another portion of the mixture of polymers and unconverted C3-C4 fraction being discharged through line 89 is directed therefrom through line 92 and valve 93 to pump 94 which discharges through line 95, valve 96, and heat exchanger 91 into header 98 containing valve 99. In heat exchanger 91, the mixture of polymers and unconverted C3-C4 fraction which is at a polymerizing temperature gives up a portion of its heat to a portion of the charged C3-C4 fraction which is directed from line 10 through branch line I09, and valve ml, to heat exchanger 91 and thence through line I02 and valve 33 to line I9 already mentioned and which is connected to heating coil 12. From heat exchanger 91, the cooled mixture of polymers and unconverted C3-C4 fraction is passed through header 98 and valve 99 and through branch line I04 to valve I05, and branch line I06 containingllli, to reactor 16 to assist, in controlling the polymerization temperature in each of the catalyst beds subsequent to the first catalyst bed. Line 14 is also connected through by-pass line I98 and valve I09 to header 98 so that the hydrocarbons charged to the process may be passed directly to the second catalyst zone in reactor 16 in case the first catalyst zone which is supported by screen 11 becomes clogged or otherwise rendered inoperaive.

Heat needed for fractionating the mixture of polymers and unconverted C3-C4 fraction in fractionator 9| is supplied by reboiler coil I i9 through which steam or some other heating fluid is supplied Irom a source not indicated in the diagrammatic drawing. Unconverted C3-C4 hydrocarbon vapors in fractionator 9i are directed therefrom throughline ii i and valve H2 to condenser 3 from which liquefied hydrocarbons or a mixture of liquefied hydrocarbons and vapors is directed through rundown line Ill and valve I I to receiver Ill provided with gas release line I I1 containing valve Ill and also with liquid drawofl. line I I9 containing valve I20. A portion of the liquefied material in receiver H8 is directed therefrom through line III and valve I22 to pump I 23 which discharges through line I24 and valve I25 to near the top of fractionator 9| to assist in controlling the temperatures therein. Polymers so separated from unconverted C3-C4 hydrocarbons in fractionator 9| are directed therefrom through line I28 and valve I21 to cooling and/or storage not indicated in the diagrammatic drawing.

The nature of the present invention is indicated further by the following example which should not be construed to limit unduly the broad scope of the invention.

A plant of the general character described in connection with Figure 1 is utilized to polymerize a C3-C4 fraction containing about 30 mole per cent of propylene and mole per cent of butylenes including isobutylene and normal butylenes. The charge is preheated to a temperature of 400 F. and passed in series through the four reactors each containing a fixed bed of 5 x 5 mm. pellets of solid phosphoric acid catalyst. A portion of the eilluent from the last catalyst reactor is partially cooled by exchanging heat with a portion of the fresh feed in heat exchanger i4 and then the cooled efiluent is commingled with the eflluent from each of the catalyst reactors before this material is passed to the next catalyst reactor in the series of four reactors indicated in Figure 1. In reactor 1 the catalyst temperature increases from 400 F. at the inlet to about 470 F. at the outlet where it is commingled with sumcient partially cooled recycle stock to lower the temperature of the mixture to about 400 F. at which temperature-the commingled mixture is introduced to the second reactor of the series. through the second reactor, the temperature of the polymerization mixture increases from 400 F. to about 475 F. due to the exothermic heat of the polymerization reaction. The effluent from the second reactor is also commingled with a portion of cooled recycle stock so that the hydrocarbon mixture charged to the third reactor of the series enters that reactor at a temperature of about 400 F. and during passage through this reactor reaohes a maximum temperature of about 480 F. due tov the heat of polymerization. The eiiluent from this reactor is then mixed with sufficient recycle stock to lower the temperature of the resultant mixture to 400 F. at which it is charged to the fourth reactor in the series of four polymerization reactors, these reactors being of increasing length in the direction of flow through the polymerization plant. The amount of polymerization reactor eilluent, including polymers and unconverted C3-C4 fraction, which is thus cooled by heat exchange with a portion of the fresh charging stock and is then recycled and commingled with the eilluent from each of the different reactors to control the temperature of the reaction mixture before introducing it to the next reactor in the series of polymerization reactors is about 0.45 times the volume of fresh C3-C4 fraction charged to'the process. Accordingly, the mixture of polymers and unconverted C3-C4 fraction which is directed from the last reactor 0f the series to the fractionating equipment has about 1.32 times the volume of the fresh C3-C4 fraction charged to the process. I

When the polymerization is carried out at a During passage pressure of 500 pounds per square inch an amount of steam corresponding to about 0.8 mole per cent of the total reaction mixture present in each of the reactors is introduced at points between the different reactors as indicated in the diagrammatic drawing in order to prevent excessive dehydration of the catalyst and accompanying loss of polymerizing activity. 7

A similar type of polymerizing treatment of a C3-C4 fraction containing 50-60 mole per cent of propylene plus butylenes is carried out in a plant of the general character referred to in Figure 2, in which the several catalyst beds of different thicknesses are contained in a single reactor although a plurality of such reactors may also be utilized. In this type of operation, a suflicient amount of the polymerization reactor eilluent including polymers and unconverted C3-C4 fraction (mainly propane and butanes) is recycled and admitted to the polymerization reactor at points in advance of each of the several catalyst beds or sections following the first catalyst bed,

in order to assist in controlling the temperatures therein. The temperature in the first catalyst bed is controlled by the amount of catalyst therein and the rate of flow of the charged hydrocarbon, mixture therethrough.

I claim as my invention:

1. A process for producing polymers from a hydrocarbon fraction containing at least about 50 mole percent of olefins of higher molecular weight than ethylene, which comprises contacting said olefin-containing hydrocarbon fraction at polymerizing conditions with a solid polymerizing catalyst contained in a series of catalyst sections of increasing thickness in the direction offlow, cooling between catalyst sections by introducing a hydrocarbon quenching stock between said catalyst sections, fractionating hydrocarbon eflluent from said catalyst sections to separate polymers and unconverted hydrocarbons, and recovering said polymers.

2. A process for producing polymers from a hydrocarbon fraction containing at least about 50 mole percent of olefins of higher molecular weight than ethylene, which comprises contacting said olefin containing hydrocarbon fraction at polymerizing conditions with a solid phosphoric acid polymerizing catalyst contained in a plurality of catalyst sections of increasing thickness in the direction of flow, cooling between catalyst sections by introducing a hydrocarbon quenching stock between said catalyst sections, fractionating hydrocarbon eiiluent from said catalyst sections to separate polymers and unconverted hydrocarbons, and recovering said polymers.

3. A process for producing polymers from a hydrocarbon fraction containing at least about 50 mole percent of olefins of higher molecular weight than ethylene, which comprises contacting said olefin-containing hydrocarbon fraction at polymerizing conditions with a solid polymerizing catalyst contained in a series of catalyst sections of increasing thickness in the direction of flow, cooling between catalyst sections by recycling thereto a portion of the total eflluent from said catalyst sections including unconverted olefin-containing fraction and polymers, separating the unrecycled eflluent into polymers and unconverted hydrocarbons, and recovering said polymers.

4. A process for producing polymers from a hydrocarbon fraction containing at least about 50 mole percent of .oleflns of higher molecular weight than ethylene, which comprises contacting said olefin-containing hydrocarbon fraction at polymerizing conditions of temperature and pressure with a solid phosphoric acid catalyst contained in a plurality of catalyst sections of progressively increasing thickness in the direction of flow, cooling between catalyst sections by recycling thereto a portion of the total eflluent from. said catalyst sections including unconverted olefin-containing fraction and polymers, separating the unrecycled eiliuent into polymers and unconverted hydrocarbons, and recovering said polymers.

5. A process for producing polymers from a hydrocarbon fraction containing at least about 50 mole percent of olefins of higher molecular weight than ethylene, which comprises contacting said olefin-containing hydrocarbon fraction at a temperature of from about 400 to about 500 F. with a solid phosphoric acid catalyst contained in a plurality of catalyst sections of increasing thickness in the direction of flow, cooling the ing a portion of thegaseous and liquid eflluent gaseous and liquid efliuent from said plurality of catalyst sections, recycling a portion of the cooled efiluent to commingle with the hydrocarbon mixture in each of the catalyst sections after the first section to assist in controlling the temperatures therein, separating the remainder of the eflluent into polymers and unconverted hydrocarbons, and recovering said polymers.

6. A process for producing polymers from a hydrocarbon fraction containing at least about 50 mole percent of olefins of higher molecular weight than ethylene, which comprises contacting said olefin-containing hydrocarbon fraction at a polymerizing temperature of from about 400 to about 500 F. with a solid phosphoric acid catalyst contained in a series of catalyst sections of increasing thickness in the direction of flow, cooling a portion of the eflluent from said series of catalyst sections by heat exchange with at least a portion of the fresh olefin-containing charging stock, recycling the cooled eilluent to each of the catalyst sections after the first section to assist in controlling the temperature therein, separating the eflluent into polymers and unconverted hydrocarbons, and recovering said polymers.

7. An olefin polymerization process which comprises reacting at C3-C4 fraction containing at least about 50 mole percent of propylene and butylenes at a polymerization temperature of from about 400 to about 500 F. in the presence of a solid calcined composite of a siliceous adsorbent and a phosphoric acid contained in a plurality of catalyst sections of increasing thickness in the direction of flow, cooling a portion of the gaseous and liquid eflluent from said plurality of catalyst sections, recycling a portion of the cooled eiiluent to commingle with the hydrocarbon mixture in each of the catalyst sections subsequent to the first section to assist in controlling the temperatures therein, separating the remainder of the eiiiuent into polymers and unconverted C3-C4 fraction, and recovering said polymers.

8. An olefin polymerization process which comprises reacting a C3-C4 fraction containing at least about 50 mole percent of olefins at a temperature of from about 400 to about 500 F. and at a pressure of from about 100 to about 2000 pounds per square inch in the presence of a solid phosphoric acid polymerization catalyst contained in a plurality of catalyst sections of infrom said plurality of catalyst sections, recycling a portion of the cooledefiluent to commingle with the hydrocarbon mixture in eachof the catalyst sections subsequent to the first section to assist in controlling the temperatures therein, introducing steam to commingle with the hydrocarbons in each of the catalyst sectionsto maintain the hydration and activity of the catalyst, separating the remainder of said eilluent into polymers and unconverted C3-C4 hydrocarbons, and recovering said polymers.

9. A process for producing polymers from a hydrocarbon fraction containing more than about 35 mole percentof olefins of higher molecular weight than ethylene, which comprises passing said fraction at polymerizing temperature through a series of beds of solid polymerizing catalyst of increasing thickness in the direction of flow of the hydrocarbons therethrough, cooling the hydrocarbons between successive catalyst beds by commingling a hydrocarbon quenching medium therewith, and fractionating efiluent products from the last catalyst bed of the series to separate polymers from unconverted hydrocarbons.

, ular weight than ethylene, which comprises passing said fraction at polymerizing temperature through a series of beds of solid polymerizing catalyst of increasing thickness in the direction of flow of the hydrocarbons therethrough, cooling the hydrocarbons between successive catalyst beds by commingling therewith a portion of the eilluent from the last catalyst bed of the series, said portion of the eiiluent containing polymers and unconverted hydrocarbons, and fractionating another portion of said efliuent to separate its polymer content from unconverted hydrocarbons.

11. A process for producing polymers from a hydrocarbon fraction containing more than about 35 mole percent of olefins of higher molecular weight than ethylene, which comprises passing said fraction at polymerizing temperature through a series of beds of solid polymerizing catalyst of increasing volume in the direction of flow of the hydrocarbons therethrough, cooling the hydrocarbons between successive catalyst beds by commingling a hydrocarbon quenching medium therewith, and fractionating eflluent products from the last catalyst bed of the series to separate polymers from unconverted hydrocarbons.

12. A process for producing polymers from a hydrocarbon fraction containing more than about 35 mole percent of olefins of higher molecular weight than ethylene, which comprises passing said fraction at polymerizing temperature through a. series of beds of solid polymerizing catalyst of increasing volume in the direction of flow of the hydrocarbons therethrough, cooling the hydrocarbons between successive catalyst beds by commingling therewith a portion of the eiiiuent from the last catalyst bed of the series, said portion of the effluent containing polymers and unconverted hydrocarbons, and fractionat- 11 terlzed in that said polymerizing catalyst is a solid phosphoric acid catalyst.

14. The process of claim 12 further characterized in that said polymerizing catalyst is a solid phosphoric acid catalyst.

WILLIAM B. SHANLEY.

REFERENCES CITED The following references are of record in the me of this patent:

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